It is well known that rays emanating from a point in the focal plane of a lens are collimated and that parallel rays incident on a lens are concentrated to a point. A flat panel lens collimates rays emanating from a point but needs no volume for rays to fan out between the point and the plane of ray collimation. In a flat panel lens, however, the point and plane periphery are separate and there is an undesirable margin inbetween the point and plane periphery.
There is space between an ordinary lens and each of its focal planes and this can also make optical systems bulky. One proposed solution discloses a flat panel lens that collimates rays emanating from a point, or from a point to a flat panel lens, within a slim tapered light guide. The focal point and exit surface are approximately coplanar but displaced from one another because a slab of light guide is disposed between the two in order that rays can fan-out from the focal point to the exit surface.
Rear projection televisions are normally bulky because space is required between the video projector and diffusive screen in order that the picture fills the screen. The space may be replaced by a flat panel lens but there is no need to collimate the light because the screen necessarily destroys collimation. Thus, a simple tapered optical waveguide with uniform cross-section can suffice. However, a slab of light guide within which rays from the projector fan-out forms a blank margin on the screen where users would rather want to see the picture fill the screen.
Alternatively, a pair of prisms can be used to fold the slab behind the screen, but the slab, tapered waveguide and prisms must be carefully aligned and this has the potential to be costly. In yet another attempted solution, the direction of rays may be reversed in all linear optical systems and a tapered light guide may be used to remove the space between a camera and the object being photographed. However, a slab of light guide is again required, this time so that rays can fan-in. Although the resulting margin may be removed with folding prisms, these again, can be very costly.